Certain types of electrical switching applications require a mechanical switch which will operate properly with low and slow actuation force, is extremely reliable, has an accurately-repeatable response, and has a small actuation differential. These requirements arise perhaps most commonly in the electromechanical thermostats used for controlling heating and cooling in homes and buildings where coils of standard bimetal strips form the switch actuation elements. For many years this thermostatic switching function has been satisfactorily performed by mercury bulb switch elements.
Snap action switches satisfy these operating criteria in many ways, and of course do not have the environmental concerns which mercury bulb switch elements raise. By "snap action switch" in this context is meant a low actuation force switch which uses an internal mechanical apparatus to rapidly shift or snap the movable contact from one position to another to make or break electrical conduction between the movable contact and a fixed contact in response to moving an operating element of the switch such as a plunger or a lever from a first to a second mechanical position. Typically, these switches require only a few millimeters of movement by the operating element to change the conduction state of the switch. Such switches can safely and reliably operate at a current level of several amperes using the standard 24 VAC power which thermostats control. However, when actuated by a low and slow actuation force such as is provided by a thermostat's coiled bimetal strip, snap action switches may occasionally hang in a state between the two conducting states, or may switch so slowly between the two conducting states that unacceptable arcing can occur when entering the non-conducting state. Either condition gives rise to unacceptable reliability and predictability of operation. Furthermore, these switches frequently have unacceptably large differentials. By this is meant that the position of the operating element at which actuation of the switch to one state occurs, differs substantially from the position of the actuation element at which actuation of the switch to the other state occurs. If the differential is too large, then the temperature range which the controlled space experiences is also too large. Accordingly, the use of snap action switches in thermostat-type applications has to this point not been particularly successful.